Step 1: Frame the idea of missing mass.
Imagine building a nucleus by bringing \(Z\) protons and \((A-Z)\) neutrons together. When you weigh the finished nucleus, it is lighter than the sum you started with. That shortfall in mass is the mass defect \(\Delta m\), given by \(\Delta m = [Z m_p + (A-Z) m_n] - M\), where \(M\) is the measured nuclear mass.
Step 2: Where the mass went.
The vanished mass has not disappeared; it has been converted into energy that holds the nucleus together. By Einstein's relation \(E = mc^2\), this energy equals \(E_b = \Delta m\, c^2\). This is the binding energy: the work needed to pull the nucleus apart into free, at-rest nucleons.
Step 3: Convenient conversion.
Using \(1\,\text{u} \equiv 931.5\) MeV, the binding energy in MeV is simply \(E_b = \Delta m(\text{in u}) \times 931.5\). The greater this value (especially per nucleon), the more tightly bound and stable the nucleus is.
\[\boxed{E_b = \Delta m\,c^2 = \Delta m(\text{u})\times 931.5\ \text{MeV}}\]